Detection et distribution des beta-lactamases a spectre elargi chez les enterobacteries

[Enterobacteriaceae and beta-lactams : wild susceptibility patterns]

Four susceptibility patterns of wild types of enterobacteria against old beta-lactams including aminopenicillins, carboxypenicillins and first-generation cephalosporins were individualized during the 1980s : susceptible, penicillinase low level, cephalosporinase and a combination of penicillinase and cephalosporinase. Such indirect detection of a mechanism of resistance allowed an interpretative reading for this class of antibiotics. At the present time, seven susceptibility patterns were proposed for this family of gram negative bacilli. Nevertheless, an analysis of results in terms of MICs and diameters of inhibition zone sizes of the main bacterial species of enterobacteria, mainly obtained from the databank of European Committee on Antimicrobial Susceptibility Testing (EUCAST), compared to that observed when overproducing strains were isolated in vivo and in vitro and to the type of beta-lactamase identified and their amino acid sequences conducted to a proposal of five susceptibility patterns. The fifth wild type individualized in several enterobacteria since 2005 is related to the synthesis of various chromosomal extended-spectrum beta-lactamases (ESBL) which hydrolyze many beta-lactams including oxyimino-cephalosporins such as ceftriaxone or cefotaxime. Their expression in a wild strain is characteristic and conducted to our interest for their role as progenitors of the transferable CTM-M types. Otherwise, a medical biologist must consider the possibility of selection of a mutant with a chromosomal overproduced beta-lactamase. But within the same beta-lactam susceptibility pattern such as for Klebsiella pneumoniae and K. oxytoca or Citrobacter amalonaticus, the spectrum of inactivation will be highly variable according to the type of enzyme overproduced. Finally, a nice synergy observed between clavulanic acid and cefotaxime or ceftriaxone or even aztreonam does not mean anytime a transferable ESBL. In some cases according to the result of enterobacterial identification, the epidemiological impact will be very low, because without multidrug resistance (MDR).

Further occurrence of extended-spectrum beta-lactamase-producing Salmonella enteritidis

We describe the further occurrence of Salmonella enteritidis producing an extended spectrum beta-lactamase (ESBL) and transferring cefotaxime, ceftazidime and aztreonam resistance. In a previous communication we describe the isolation of a strain of S. enteritidis (No. 35) from a septic neonate with high-level resistance to cefuroxime, cefotaxime, ceftazidime and aztreonam. Six weeks later, a second neonate was found, in the same Pediatric University Hospital, to be infected with apparently the same strain of S. enteritidis (No. 100) which was isolated from blood with similar properties as the strain No. 35. In this strain we were able to study, in more detail, the transferability of its resistance to 3rd generation cephalosporins and its ESBL production. In this report we describe results of transfer experiments, relative rates of hydrolysis (Vmax) and ESBL tests which indicate that the capacity to produce ESBL was transferred from the multiply-resistant strain of S. enteritidis No. 100 to S. typhimurium LT-2 rif+str+.

Detection of extended-spectrum beta-lactamases in clinical isolates of Klebsiella pneumoniae and Escherichia coli

Forty clinical isolates of Escherichia coli and 141 isolates of Klebsiella pneumoniae that either transferred ceftazidime resistance or showed sulbactam enhancement of oxyimino-beta-lactam susceptibility were tested by disk diffusion methodology for susceptibility to aztreonam, cefotaxime, ceftazidime, and cefoxitin. With standard 30 micrograms antibiotic disks, the fraction of these extended-spectrum beta-lactamase (ESBL)-producing isolates testing resistant by National Committee for Clinical Laboratory Standards criteria was lowest (24%) with cefotaxime disks. Forty percent of the E. coli and 29% of the K. pneumoniae isolates appeared susceptible with at least one oxyimino-beta-lactam disk. Ceftazidime and aztreonam disks were equivalent in differentiating ESBL production, and both were superior to cefotaxime disks. Over half the E. Coli and 29% of the K. pneumoniae isolates tested cefoxitin resistant. In 30 isolates, cefoxitin resistance was transmissible and due to a plasmid-mediated AmpC-type beta-lactamase. With a 5-micrograms ceftazidime disk, a breakpoint could be chosen with high sensitivity and specificity for ESBL-producing organisms. Present disk diffusion criteria underestimate the prevalence of ESBL-producing strains.

Origin and impact of plasmid-mediated extended-spectrum beta-lactamases

Resistance to oxyimino cephalosporins was originally highlighted by the emergence of plasmid-encoded extended-spectrum beta-lactamases deriving by mutation from TEM-1, TEM-2 and SHV type enzymes (class A). The broader spectrum of resistance produced by these enzymes is related to more amino acid substitutions, but susceptibility to seven alpha-methoxyimino cephalosporins and carbapenems was preserved until recently. Clavulanate-sensitive extended-spectrum beta-lactamases are distributed worldwide, mainly among Klebsiella pneumoniae isolates. Novel clavulanate-sensitive extended-spectrum beta-lactamases deriving from other class A enzymes (e.g. MEN-1 from beta la OXY, OXA-11 in Pseudomonas aeruginosa from PSE-2) have been reported. Recently, clavulanate-resistant extended-spectrum beta-lactamases (class C) were encountered amongst single isolates, mostly Klebsiella pneumoniae. These cephalosporinases or cefamycinases (usually chromosomally mediated) have expanded the spectrum of plasmid-encoded resistance to include seven alpha-methoxyimino cephalosporins. Thus far, only two isolates (1 Pseudomonas aeruginosa, 1 Bacteroides fragilis), both recovered in Japan, with plasmid-mediated resistance to carbapenems have been found.

Resistance to third generation cephalosporins: the current situation

Newer beta-lactam antibiotics, notably the third generation cephalosporins (3 GC) have been designed for providing high intrinsic potency against a large variety of microorganisms. Bacterial resistance can occur however, and nowadays, clinicians are concerned by novel situations where even most recently developed compounds can be ineffective. A first situation is generated by bacteria which produce great amounts of chromosomal cephalosporinase. The resistance emerges during therapy, in hospital isolates which are classified as susceptible with conventional susceptibility testing. The prevalence of 3 GC resistance among these gram-negative rods with inducible beta-lactamase seems to increase in some institutions but the significance of susceptibility testing in this regard is doubtful. It is probably more important to note that the prevalence of gram-negative rods with inducible beta-lactamases remains stable. A second problem arose with the abrupt development of plasmid mediated beta-lactamases markedly active against 3 GC. This resistance is underestimated because some strains fall into susceptibility range of 3 GC as determined by MICs or inhibition zone sizes. These extended spectrum enzymes are now distributed over four continents and represent a growing threat.

Epidemiology of extended spectrum beta-lactamases

Beta-lactamases play a major part in resistance, as recently redemonstrated by the emergence of extended spectrum beta-lactamases. Since its discovery in FR Germany, SHV-2 has been reported from four continents and CTX-1 (TEM-3) was established in at least 26 French hospitals. More than 12 other enzymes have been individualized. The newest aspect of resistance was probably underestimated because most strains of enterobacteria (mainly Klebsiella pneumoniae) appeared susceptible to oxyimino-beta-lactams as suggested by MICs or diameters of inhibition zone sizes. The double-disk synergy test between amoxicillin/clavulanic acid and oxyimino-beta-lactams was useful to easily detect two susceptibility patterns (CTX, CAZ). Extended spectrum beta-lactamases isolated among nosocomial isolates of enterobacteria (urines, blood, wound, sputum cultures) mostly from intensive care units have spread through hospitals. If outbreaks were described, numerous serotypes were identified in Klebsiella pneumoniae. In France the distribution of extended spectrum beta-lactamases showed that CTX-1 (TEM-3) was well distributed among ten species unlike SHV-type enzymes (SHV-2, SHV-3, SHV-4) preferentially detected in Klebsiella pneumoniae. A majority of strains produced CAZ-type enzymes in Escherichia coli. Some isolates produced two extended spectrum beta-lactamases. In Tunisia extended spectrum beta-lactamase producing strains were mainly identified among pediatric isolates of Klebsiella pneumoniae, Salmonella and Escherichia coli; SHV-2 was predominant but recently CTX-1 and two other types with an isoelectric point of 6.35 and 5.4 (phenotype CTX) were individualized. Because plasmid-encoded, this mechanism was spreading in France among enterobacteria with other resistance markers (e.g. netilmicin, amikacin) for CTX-1 unlike SHV-2.(ABSTRACT TRUNCATED AT 250 WORDS)